The present invention relates to apparatus for measuring the propagation time of ultrasonic waves. More particularly, the invention relates to apparatus including a wave transmitter and a wave receiver arranged in separated and opposed relationship with each other, which apparatus measures the time interval from the transmission of an ultrasonic pulse by the transmitter until the receiving of such pulse by the receiver and calculates wind velocity or flow velocity on the basis of the result of the measurement.
In apparatus of the aforedescribed type, the time interval from the transmission until the reception of a pulse is measured by counting the number of clock pulses within said interval. In conventional apparatus, however, only an integral number of clock pulses are counted, and any time of less than one pulse is neglected and omitted. However, in apparatus of this type, the time interval between the transmission of a pulse and its reception is extremely short, so that any time of less than one clock pulse produces an error which can not be neglected in practice. Since there is no appropriate solution of this problem, however, the error inevitably appears in the measured result.
One method of improving the measurement accuracy would be to increase the frequency of the clock pulses. However, this method would result in a complicated structure of each of the transmitter, receiver, counter, etc., and the inspection and maintenance of these devices would be time-consuming and troublesome.